1. Field of the Invention
The invention relates to a frequency-modulated continuous wave (FMCW) radar system for detecting the distance or range, the azimuth angle and/or the relative or radial velocity of a reflection object or target object in the field being monitored.
2. Description of the Prior Art
FMCW radar systems are well-known and wide spread for use in vehicle collision avoidance and tracking cruise keeping a constant distance to the vehicle ahead. In FMCW radar systems, a transmission signal is so frequency-modulated as to gradually or linearly increase and decrease in frequency. The frequency-modulated transmission signal is transmitted as a radar wave and reflected by a reflection object or target. The reflected or returned radar wave from the target is mixed with the transmission signal to yield a beat signal. The beat signal is analog-to-digital converted and Fourier converted. The peak frequencies of the beat signal are identified as to which each of the peak frequencies belongs to, the increasing section or the decreasing section of each modulation period. On the basis of a peak frequency fpI in the increasing section and a peak frequency fpD in the decreasing section, the distance D and the relative velocity V to the target (referred to en bloc as “target information”) are calculated by using the following equations:                     D        =                              C                          8              ⁢                                                           ⁢              Δ              ⁢                                                           ⁢                              F                ·                fm                                              ·                      (                          fpI              +              fpD                        )                                              (        1        )                                V        =                              C                          4              ⁢              f0                                ·                      (                          fpI              -              fpD                        )                                              (        2        )            where ΔF is the range of variation in the transmission signal, f0 is the central frequency of the transmission signal, 1/fm is the period of time required for one cycle of modulation, and C is the velocity of light.
In actual operation environment, several target objects often exist in the field being monitored, which causes the same number of peak frequencies as that of the target objects to be detected in each of the increasing and decreasing sections.
It is well known that if the relative velocities of a vehicle ahead and a vehicle mounted with the FMCW radar system are different from each other: i.e., V=0 in equation (2), then the peak frequencies fpI and fpD for the vehicle ahead are shifted due to the Doppler effect. Accordingly, if there are two or more targets in the monitor field, the peak frequencies for the targets do not always appear in order of distances to the targets. In selecting, for each target, a corresponding peak frequency from each of the increasing and decreasing sections of a modulation cycle, simply taking one by one from each section in order of distances to the targets may yield wrong values of the distance, the azimuth angle and the relative velocity.
For this reason, whether the peak frequencies in the increasing section and the peak frequencies in the decreasing section have been properly paired for respective targets is judged in the following manner. i.e., (1) the peak frequencies in the increasing section and the peak frequencies in the decreasing section are pieced together in an appropriate way to form provisional peak pairs associated with the respective targets; (2) on the basis of the behavior or movement of a target associated with each of the provisional peak pairs, a pair of peak frequencies to be detected after the lapse of a certain period are estimated for the target; (3) if any peak pair which substantially matches the estimated pair of peak frequencies (referred to as “estimated peak pair”) is found in peak frequencies actually detected after the lapse of the certain period, then it is judged that the provisional peak pair is a true peak pair.
If, as the behavior of a target associated with a provisional peak pair (referred to as “provisional target”), it is found that the target is running 100 m ahead at a speed of 50 Km/h for example, then the target is estimated to be running about 113.9 m ahead at 50 Km/h after the lapse of a certain period, e.g., 1 second. Accordingly, what has to be done is to find a peak pair that shows such a behavior in the peak frequencies detected after the lapse of the certain period (after 1 second).
However, just described peak pair identification method such as judges whether a provisional peak pair is right or not on the basis of the behavior of a provisional target associated with the provisional peak pair causes it to take time to output the target information, and accordingly is not suitable for the recognition of rapidly approaching target or an application that needs quick recognition.
Specifically, as is well known in the art, the distance resolution of FMCW radar system depends on the modulation width of the radar wave. In order to enable a correct estimation of the behavior of a provisional target, the provisional target has to make a move of distance more than the distance resolution. The time required for the move is set as an initial detection period (Dr) of time in FMCW radars. The shorter initial detection period enables the quicker recognition. However, it is necessary to set the initial detection period to a value larger than Dr/Vmin, where Vmin is the lower limit value of the relative velocity which enables the detection of the target. As seen from this, trying to raise the detection precision for lower-relative-velocity targets results in a longer initial detection period. Taking a margin into account in order to enhance the recognition reliability will cause the initial detection period to become further longer.
In view of the above, what is needed is a FMCW radar system provided with both a raised detection accuracy for lower-relative-velocity targets and a quicker response in detection of higher-relative velocity targets.